Journal
INTERNATIONAL JOURNAL OF CHEMICAL KINETICS
Volume 47, Issue 7, Pages 420-428Publisher
WILEY
DOI: 10.1002/kin.20919
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Funding
- Swedish Research Council (Vetenskapsradet) [2011-3921]
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Reaction with hydroxyl radicals (OH) is the major pathway for removal of cyclic volatile methyl siloxanes (cVMS) from air. We present new measurements of second-order rate constants for reactions of the cVMS octamethylcyclotetrasiloxane (D-4), decamethylcyclopentasiloxane (D-5), and dodecamethylcyclohexasiloxane (D-6) with OH determined at temperatures between 313 and 353 K. Our measurements were made using the method of relative rates with cyclohexane as a reference substance and were conducted in a 140-mL gas-phase reaction chamber with online mass spectrometry analysis. When extrapolated to 298 K, our measured reaction rate constants of D-4 and D-5 with the OH radical are 1.9 x 10(-12) (95% confidence interval (CI): (1.7-2.2) x 10(-12)) and 2.6 x 10(-12) (CI: (2.3-2.9) x 10(-12)) cm(3) molecule(-1) s(-1), respectively, which are 1.9x and 1.7x faster than previous measurements. Our measured rate constant for D-6 is 2.8 x 10(-12) (CI: (2.5-3.2) x 10(-12)) cm(3) molecule(-1) s(-1) and to our knowledge there are no comparable laboratory measurements in the literature. Reaction rates for D-5 were 33% higher than for D-4 (CI: 30-37%), whereas the rates for D-6 were only 8% higher than for D-5 (CI: 5-10%). The activation energies of the reactions of D-4, D-5, and D-6 with OH were not statistically different and had a value of 4300 +/- 2800 J/mol.
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